The MPC-EX detector is a Si-W preshower extension to PHENIX's existing Muon Piston Calorimeter (MPC).   It consists of eight layers of alternating W absorber and Si mini-pad sensors.  The MPC-EX, in conjunction with the MPC, can measure high energy neutral pions and direct photons.  Located at forward rapidity, 3.1 < |\eta| < 3.8, the MPC and MPC-EX will access low-x partons in the nucleus in p+A collisions.  At forward rapidities, prompt photons are dominated by direct photons produced when a valence quark in the projectile scatters off of a gluon in the target.  In p+A collisions, the prompt photon $R_{pA}$ will quantify the level of gluon saturation in the nucleus at low-x, $x ~ 10^{-3}$, with a projected systematic error band a factor of four smaller than EPS09’s current allowable range.  When the proton in p+A collisions is transversely polarized, a unique capability of RHIC, the resulting single spin asymmetries can probe the saturation scale in a Color Glass Condensate picture.  This talk will present more details on the MPC-EX detector and its capabilities to expand our understanding of gluon nuclear parton behavior at low-x and provide important information about the initial state of heavy ion collisions.